How to make bones follow each other instead of individually?
I agree with Frederik's comment on your original post, using either the spline IK or bendy bones would be how I would approach the problem if you wanted to have animation control over the tentacles. For the flailing, you could parent the IK targets to empties that are randomly animated using noise/sinusoidal modifier on the animation curves or the "wiggle" feature in animation nodes. 

Another way to approach it if you don't care about being able to pose the tentacles could be to use cloth physics for each arm as is done for a rope in this post: How To Simulate a Rope. You could use hooks as is done in the tutorial, or alternatively use a wind and turbulence force field on the "ropes" to get them to wiggle randomly. In the example below, i used the skin modifier on the cloth "string" to give the tentacles some mass and a subsurf modifier, then duplicated the objects around the hole. The wind and turbulence fields give them the "wiggle".

Of course this is just one suggestion. You could combine methods and have one or two you animate and the others random. Hope this is helpful?